1. Field of the Invention
This invention relates to isocyanate-terminated polyurethane prepolymers based on isocyanates of different reactivity, and a process for their production. The products obtained by this process are distinguished by their reduced content of free monomeric diisocyanates.
2. Statement of the Related Art
Isocyanate-terminated polyurethane prepolymers have long been known. They may be readily reacted with suitable hardeners, generally polyhydric alcohols, to form high polymers. Polyurethane prepolymers have acquired significance in numerous fields, including sealing compounds, lacquers and adhesives.
To obtain isocyanate-terminated polyurethane prepolymers, it is standard practice to react polyhydric alcohols with an excess of diisocyanates. As is generally known among people skilled in the field of polymer chemistry, the average molecular weight can be controlled at least approximately through the ratio of hydroxyl groups to isocyanate groups in this reaction. Thus, products of very high molecular weight are formed where the molar ratio in question is exactly 1:1, whereas lower molecular weight adducts of 2 molecules of diisocyanate with 1 molecule of diol are formed on a statistical average where the molar OH:NCO ratio is 1:2. On the strength of this knowledge, it is possible for one to "tailor" isocyanate-terminated polyurethane prepolymers having molecular weights variable over wide limits. However, the products formed, like most polymers, have a more or less wide molecular weight distribution. In particular, a certain quantity of the component used in excess is left over unreacted on completion of the reaction, irrespective of the reaction time. The content of unreacted diisocyanates (hereinafter referred to as residual monomers) increases with the excess of that component in the reaction mixture (cf. H. G. Elias, "Makromolekule", Huthig & Wepf, Basel, 4th Edition 1981, pages 487, et seq.).
In numerous applications of polyurethane prepolymers, problems are presented by the presence of residual monomers. Thus, volatile diisocyanate monomers such as tolylene diisocyanate necessitate special precautionary measures concerned with industrial safety, even if they are present in the prepolymers in quantities of only 0.5 to 5% by weight. On the other hand, involatile excess diisocyanates can give rise to problems through migration in polyurethane adhesives. The problems in question can arise, for example, in the sealing of bonded laminates.
To reduce the content of residual monomers, it has been proposed in earlier filed copending German Patent Application P No. 33 06 559.4, and corresponding U.S. Pat. No. 4,487,910 to prepare a low viscosity polyurethane prepolymer from a monocyclic diisocyanate and a polyhydric alcohol and to react a dicyclic diisocyanate with an alcohol component in that polyurethane prepolymer as solvent or reactive diluent. Although low-monomer polyurethane prepolymers can be obtained by that process, it is nevertheless desirable to have products which combine a low residual monomer content with a low viscosity.